Conversion of waste into highly efficient fuel

ABSTRACT

Solid combustible waste materials are converted into highly efficient fuel by subjecting such materials to size reduction in suitable size-reducing equipment. The last piece of the equipment is a mill which pulverizes the waste materials into fine particles having a high surface to mass ratio and forming a highly efficient fuel when these particles are directly injected into a combustion reactor operating at high temperature.

FIELD OF THE INVENTION

This invention relates to a method and an apparatus for converting solidcombustible waste materials, such as paper, cardboard, food, plastics,textiles, wood and the like, into fine particles which form a highlyefficient fuel when fed directly into a combustion reactor, particularlysuch as a plasma arc waste destruction furnace.

BACKGROUND OF THE INVENTION

Typically, combustible solid waste is introduced into a thermaltreatment furnace, such as an incinerator, in its original form or afterbeing reduced in size by a shredder-type device. The moisture content ofthe waste particles is usually “as-received” and the smallest practicalsize achieved by a conventional shredder is several centimetres.

Efforts have also been made in the past to convert waste materials, suchas waste paper products, into useful forms, including fuel. One suchmethod is disclosed in U.S. Pat. No. 4,123,489 where paper waste isprocessed by a rotary cutter which includes a knife cylinder having aplurality of blades for cutting the waste paper products fed into themachine into smaller pieces or particles. The cutter includes a recutterscreen having a surface cooperating with the periphery of the rotatingknife cylinder, providing sizing openings for further reducing the sizeof the pieces of paper waste. The pieces passing through the recutterscreen are cut to a maximum of ⅛ inch by 2 inches which makes thematerial suitable for various purposes, including feeding into the diecavity of a pelletizing machine to form high quality, relativelydust-free pellets of paper material, that can be used as a fuel. Suchpellets, however, do not constitute a very efficient fuel since theirsurface to mass ratio is not very high.

There is thus a need for the conversion of solid combustible waste intoa highly efficient fuel that can be readily used, for example, in aplasma fired eductor or any other combustion reactor.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and anapparatus for the conversion of combustible waste into highly efficientfuel.

It is a further object to provide a solid fuel stream from such waste,which will easily burn in a plasma furnace or other combustion reactoror incinerator.

Other objects and advantages of the invention will be apparent from thefollowing description thereof.

The invention, reported herein, is based on the concept of convertingwaste into a fuel for efficient combustion in a thermal treatmentsystem. A fuel, for the purpose of this invention, is defined as acombustible material which has been milled to dramatically increase itssurface area to mass ratio and dried to a moisture content of less than5% by weight.

The waste treatment system of the present invention subjects combustiblewaste, which includes materials such as paper, cardboard, food,plastics, textiles and wood, to size reducing steps achieved by suitablesize reducing equipment leading to a finely pulverized product. Thefinal pulverized product is in the form of fine particles or fibershaving a high surface to mass ratio

Such particles, which usually have a diameter of 15 μm or less, are fedpneumatically to a desired type of combustion reactor without anyintermediate transformation into pellets or the like. This directconveying of the fine particles into a combustion reactor, such as anincinerator or a waste treatment furnace, or a high-efficiencyplasma-fired eductor of a plasma arc waste destruction system, allowsthem to gasify rapidly when exposed to the high heat of the reactor(about 1000° C. or higher), thus significantly increasing combustionefficiency.

In essence, in accordance with the present invention, a stream of solidcombustible waste is converted into a solid fuel stream consisting offinely pulverized waste material which is then fed into a combustionreactor operating at high temperature adapted to rapidly gasify thefinely pulverized material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the appendeddrawings in which:

FIG. 1 is a schematic block diagram, illustrating the operation of anembodiment of the present invention; and

FIG. 2 is a perspective view of equipment used within the apparatusproducing the operation illustrated in FIG. 1.

DETAILED DESCRIPTION

A preferred embodiment of the invention is illustrated in FIG. 1.According to this embodiment, waste can be subjected at 10 to pulpingfollowed by water removal and/or at 12 to shredding followed by metalextraction. Pulping, mainly of food waste, is carried out in a pulperwhere the size of the particles is reduced to a size suitable formilling into fine particles, which is usually to less than ½ cm.Following the pulper, water is removed to yield an extracted pulpedproduct containing a predetermined amount of solids, e.g. approximately50% solids, by weight.

Mixed waste, including paper, cardboard, food, plastic, wood and textilewastes, is subjected at 12 to size reduction and extraction of any metalthat may be present in such waste. This can be done, for instance, in ashredder where the size of such waste is reduced to small piecessuitable for milling into fine particles, for example of about 2.5 cm insize.

Once the waste materials have been reduced at 10 and/or 12 to a sizesuitable for milling, they are subjected to milling at 14 where the sizeof the waste is pulverized to fine fibers or particles, preferablyhaving a diameter of about 15 μm or less, and the moisture content isreduced in the mill from about 50% to less than 5% by weight, whichrepresents an essentially dry condition. Such fine particles have a highsurface to mass ratio and form a highly efficient fuel. Air is added tothe mill to act as a carrier for the pulverized waste which can then bepneumatically fed through conduit 16 to a combustion reactor 18, whichcan be an incinerator, a plasma treatment furnace, a plasma firedeductor, or the like.

FIG. 2 illustrates the equipment suitable for the purposes of thepresent invention. Pulper 20 is provided to treat primarily food waste,but which may also contain some paper, cardboard and other pulpablematerials. In this pulper, the waste is normally reduced to a size ofless than 0.5 cm and the slurry exiting the pulper by conduit 22 andcontaining approximately 1% by weight of solids, enters a waterextractor 24 where water is removed by mechanical means to yield aproduct in the from of pressed pulp that contains approximately 50%solids by weight. This pressed pulp is then fed onto conveyor 26 andfrom this conveyor to a hopper/mixer 28 where it is kept in admixturewith other waste materials coming from a shredder 30.

Mixed waste, which may contain paper, cardboard, food, plastics, woodand textiles, is fed into the shredder 30 where its size is reduced to adegree suitable for milling into fine particles, for example in theneighbourhood of 2.5 cm. Such shredded waste is then conveyed via asuitable conveyor 32 to a metal extractor 34 which eliminates anymetallic matter that may have been present in such waste. This can bedone by passing the shredded waste through a suitable screen that willcatch larger metallic pieces as well as by using magnets to removemagnetic materials and other suitable means. From the metal extractor34, the shredded waste is fed to the conveyor 26 to be mixed withpressed pulp. This conveyor 26 is normally an auger with cut and foldedflights which mixes the material as it is conveyed to the hopper/mixer28. The mixed waste is metered from the hopper/mixer 28 into a mill 35via a rotary valve 36. In the mill 35, the size of the waste is reducedto fine fibers or particles, preferably of about 15 μm or less indiameter and the moisture content is reduced from about 50% to about 4%by weight. The mechanical work performed by the mill 35 in pulverizingthe waste, also performs the drying of the waste. Air is added to themill 35 via conduit 38 to act as a carrier for the pulverized wastewhich is then fed pneumatically via conduit 40 to a combustion reactor42. In this case, the combustion reactor 42 consists of a plasma arcwaste destruction system and the pulverized waste is fed into theplasma-fired eductor 44 at the inlet thereof The pulverized waste isfully combusted in this system to produce CO₂ and H₂O at the outlet 46.

The invention is not limited to the specific embodiments describedabove, and includes various modifications obvious to those skilled inthe art, without departing from the scope of the following claims.

1. Method of converting solid combustible waste materials into highlyefficient fuel, which comprises subjecting the waste materials to sizereducing steps of which the last step comprises a milling operationwhich pulverizes the waste materials into fine particles having a highsurface to mass ratio and forming a highly efficient fuel, andpneumatically feeding said fine particles into a combustion reactoroperating at high temperature adapted to rapidly gasify said particles.2. Method according to claim 1, in which the waste materials includepaper, cardboard, food, plastics, textiles and wood.
 3. Method accordingto claim 1, in which prior to the milling operation, mixed waste issubjected to shredding, followed by metal extraction, to produce ashredded metal-free product of a size suitable for milling into fineparticles.
 4. Method according to claim 1, in which prior to the millingoperation, primarily food waste is subjected to pulping, followed bywater removal, to produce a pulped product having a particle sizesuitable for milling into fine particles.
 5. Method according to claim1, in which primarily food waste is subjected in a size-reducing step topulping, followed by water removal to produce a pulped product having aparticle size suitable for milling into fine particles, and mixed wasteis subjected in a separate size-reducing step to shredding, followed bymetal extraction of any fugitive metallic materials, to produce ashredded, metal-free product of a size suitable for milling into fineparticles, said pulped product and said shredded metal-free product arethen intermixed and their mixture is subjected to the milling operationto produce essentially dry fine particles having a high surface to massratio and forming a highly efficient fuel, which particles are thendirectly fed into the combustion reactor.
 6. Method according to claim1, in which the milling operation is adapted to pulverize the wastematerials to a particle size of 15 μm or less in diameter.
 7. Apparatusfor converting solid combustible waste materials into highly efficientfuel, which comprises suitable size reducing equipment ending with amill capable of pulverizing the waste materials into fine particleshaving a high surface to mass ratio and forming a highly efficient fuel,and means for pneumatically transferring said fine particles directlyinto a combustion reactor.
 8. Apparatus according to claim 7, in whichinitial size reducing equipment, used prior to the mill, comprises apulper where a slurry is formed and the size of the waste material isreduced to a particle size suitable for milling.
 9. Apparatus accordingto claim 8, in which the pulper is followed by a water extractor andmeans being provided for feeding said slurry into said water extractorin which water is removed to achieve a pulped material having apredetermined amount of solids.
 10. Apparatus according to claim 7, inwhich initial size reducing equipment, used prior to the mill, comprisesa shredder where the size of the waste material is reduced to a degreesuitable for milling into fine particles.
 11. Apparatus according toclaim 10, in which the shredder is followed by a metal extractor forremoving any fugitive metallic pieces and means being provided forpassing shredded waste material through the metal extractor after saidmaterial exits the shredder.
 12. Apparatus according to claim 7, furthercomprising means for intermixing the waste materials prior to feedingthem into the mill.
 13. Apparatus according to claim 7, in which themill is adapted to mill the waste materials to form essentially dryparticles of a size of about 15 μm or less in diameter.
 14. Apparatusaccording to claim 7, in which means are provided for injecting air intothe mill so as to allow transfer of the fine particles produced in themill by pneumatic means into a combustion reactor.
 15. Apparatusaccording to claim 7, in which the combustion reactor is a plasmafurnace or a plasma-fired eductor of a plasma waste destruction system.